Genotoxic chemicals are an existing wide-spread health hazard to the human population. Advances in genetic toxicology testing have made it possible to assay potential mutagens, carcinogens, teratogens and clastogens in the environment. However, many currently available mammalian cell tests are costly, time consuming and labor intensive. This research program is directed toward the application of high-speed flow cytometry (FCM) to expedite genetic toxicology testing using mammalian cell systems. The flow cytometer is able to analyze more than 100,000 cells/minute, and thus should be able to provide 1) faster analysis of target cells, 2) larger populations of cells for better statistics, 3) the potential for automating mammalian cell tests. Our Phase II program will focus on experiments designed to optimize and refine the micronucleus assay procedures that were developed during Phase I. The FCM-methodology will also be validated with ten standard clastogens. Finally, FCM methods will be used to expedite the analysis of mutations at select target genes in mammalian cells. The procedures and methods will be designed for routine commercial application of the assays.